Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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FXUS61 KPHI 140934
AFDPHI

Area Forecast Discussion
National Weather Service Mount Holly NJ
534 AM EDT Mon May 14 2018

.SYNOPSIS...
A front to the south of the area will slowly move northward
through the northern Mid-Atlantic today and tonight. The front
will likely dissipate as another cold front approaches the
region on Tuesday. This front will stall just to the south on
Wednesday and remain in close proximity through the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Challenging forecast today, as the front to the south of the
area is expected to move slowly northward today, bringing warmer
and moister air with it. The problem is the timing, with quite
a bit of remaining disagreement amongst the deterministic
models. I suspect the consensus is on the overly aggressive
(i.e., faster-moving front) side, so I weighted the slower
deterministic guidance a little bit higher.

The result of this is that I decreased maximum temperatures
across the region today a few degrees, especially north of I-78,
where it will take most of the day for the warmer/moister air
to make it. It is quite possible temperatures are overdone still
in this area, with errors of several degrees possible.
Additionally, implications on sky cover are high, with a slower
frontal progression resulting in cloudier conditions for a
longer period of the day. Currently, forecast highs are in the
70s for most of the area, with some 60s likely in the higher
elevations of the northern CWA, and some low 80s in southern
Delaware and adjacent portions of Maryland.

Meanwhile, the showers/storms that passed through earlier have
effectively mixed the low levels, so fog has been fairly sparse
across the area. However, some of the more susceptible spots
have tanked in the past couple of hours, especially north of
I-78 where less precipitation occurred. Kept patchy fog/drizzle
in the forecast this morning, but I expect the impacts to be
fairly limited.

Today should be generally dry, as no midlevel perturbations
will be in proximity to the area during this time. This will be
short-lived, however, as the discussions below will explain.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/...
The next in the series of midlevel perturbations will be
approaching the Mid-Atlantic tonight, with a convective system
expected to develop in the lower Great Lakes and Ohio Valley
today, moving southeastward through this evening. The track of
this convective system is unclear, with the NAM taking it well
southwest of the area by this evening. Propagation effects would
be at work here, as the convection initiates progressively into
increasingly unstable air. However, the front`s progression
northward suggests that a more eastward track is possible, as
depicted by the GFS. Though I lean toward the convection-
allowing models (with the convection generally missing the
area), the large-scale pattern is not overly convincing of the
hi-res convective system tracks (and there is large disagreement
model to model in this respect). Therefore, cannot rule out
storms this evening anywhere in the CWA, so kept slight chances
in after sunset, though I would expect the convection to be on
the wane as nocturnal cooling sets in.

Yet another perturbation approaches the area in a more west-
to- east trajectory late tonight. Models are really all over the
place with the attendant precipitation, with the NAM Nest
developing some convection in northern/central PA but
essentially dissipating it (or keeping it well north of the
area). This solution has support from the WRF-ARW, but the WRF-
NMM indicates some of this precipitation could make it to the
northern/western CWA before dissipating. The 00Z NAM progresses
the precipitation even farther north (but again, this will
depend on the frontal position, which is uncertain). Meanwhile,
the GFS tells a completely different tale, with precipitation
readily making it into the area after midnight.

With such large disagreement displayed in the deterministic
model suite, kept slight-chance to low-chance PoPs across the
area tonight. Elevated instability would be available (much like
tonight), so thunder chances were included. However, it is
quite possible the entire area remains dry. Given the large-
scale pattern in place, though, it seems somewhat foolhardy to
submit a dry forecast at this point.

Lows tonight will be warmer as warm/moist advection encompasses
the area. Current forecast has 60s almost everywhere, with
perhaps a couple sub-60 readings in the far north.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
A very unsettle weather pattern will continue right through the
upcoming week and into next weekend with seemingly endless
chances for showers. Flooding concerns increase as a result with
particular concern for localized heavy rainfall/flash flooding
Tuesday late afternoon and night.

Tuesday and Tuesday night...The next synoptic cold front will
quickly approach from the N/W. This front should move through
the area sometime during the afternoon and night on Tuesday once
a weak wave of low pressure along this boundary passes to our
north. Unfortunately, the NW push in wake of this surface low
will be weak, leading to yet another situation where the front
may stall over the area and bring the risk of heavy
rainfall/flooding and strong to severe storms on Tuesday
(currently, this looks like another late day into the night
event). Both threats (especially severe thunderstorm) in terms
of severity and geographical coverage will be conditional on
sufficient daytime heating occuring in the pre- frontal warm
sector. For most of the CWA there is the potential for severe
thunderstorms.

Wednesday through Friday... The cold front looks to stall
nearby to our south for the second half of the work week. Maybe
some locations could squeeze out a dry period later Wednesday-
early Thursday (especially northern zones) but with an endless
train of fast-moving shortwave disturbances passing through the
Mid-Atlantic region and interacting with the stationary
boundary. Even though it may be an extended rainy period
(especially late week), the more measurable risk for strong
thunderstorms and heavy rainfall/flash flooding will be
suppressed to our south given this boundary remains just to our
south as model consensus has trended over the past several runs.
However, there is still a potential for the front to drift back
northward, particularly for our Delmarva zones.

Friday night through Sunday... Downstream blocking from a
strong Bermuda high will continue into the weekend. However,
with the other high to our north appearing to finally weaken
over Canada and eventually retreat offshore, the stalled front
could finally be able to lift north of our area. Unfortunately,
the pattern still looks unsettled with a weak cutoff low over
the northern Gulf Coast forecast to open up and drift northward
along the western periphery of the Bermuda ridge. Additionally,
another cold front will approach from the N/W during the
weekend. A more favorable environment for convection in the warm
sector will yield an increased risk for thunderstorms and
locally heavy rainfall.

&&

.AVIATION /10Z MONDAY THROUGH FRIDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

Rest of tonight...IFR/LIFR conditions should prevail, though
there looks to be a period of MVFR CIGs and VFR VSBYs
immediately upstream of the showers/storms that have passed
through. Fog/drizzle should redevelop with time, however, as
CIGs become solidly IFR or worse by 08Z. Winds light and
variable. Overall confidence is rather low given the lingering
effects of the showers/storms that passed through.

Monday...IFR conditions in the morning are expected to improve
slowly to MVFR and possibly VFR by late afternoon. However, the
timing of this is very uncertain, as forecast models tend to
project these improving conditions too fast. Trended the
forecast a little slower for the 06Z TAFs, and this may not be
enough. Winds will gradually become south or southwest around 5
to 10 kts by afternoon. Low confidence.

Monday night...MVFR/VFR conditions may deteriorate to IFR, with
at least a chance of patchy fog. However, there is a slight
chance of showers, which may hinder more widespread fog
formation. Winds light and variable. Low confidence.

Outlook...
Tuesday morning...MVFR/IFR conditions improving to VFR.

Tuesday afternoon...An increasing chance of showers and
thunderstorms, some with heavy rain and gusty winds. VFR
lowering to MVFR/IFR in precipitation.

Tuesday night...MVFR/IFR conditions. Showers and thunderstorms
likely in the evening with heavy rain and gusty winds possible.
A chance of showers overnight.

Wednesday and Wednesday night...MVFR/IFR. A chance of showers.

Thursday...VFR/MVFR. A chance of showers.

Thursday night and Friday...MVFR/IFR with showers likely.

&&

.MARINE...
Sub-advisory winds/seas are expected through tonight. There may
be some patchy fog early this morning as the showers/storms
move well offshore, with locally reduced visibilities. Southerly
flow will return as a warm front slowly moves through the area
through tonight, with no precipitation expected today. However,
there is a slight chance of storms tonight as another system
approaches the area.

Outlook...
Tuesday...No marine headlines are anticipated.

Tuesday night...No marine headlines are anticipated. However,
strong thunderstorms may cause brief wind gusts in excess of 30
knots.

Wednesday through Friday...No marine headlines are anticipated.

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...None.

&&

$$

Synopsis...CMS
Near Term...CMS
Short Term...CMS
Long Term...Iovino/Klein
Aviation...CMS/Iovino
Marine...CMS/Iovino



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